Finnmark FD-5 Trinity XL Infrared & Steam Sauna | 4-Person | Cedar & Thermal Aspen

Fit

Inside the Finnmark FD-5 Trinity XL: benches 70.5 inches wide, on two tiers

Four adults and a stove need room, and this is where Finnmark puts it. The upper bench alone is 70.5 by 20.1 inches, wider than the whole interior of the two person Trinity.

Inside the cabin

Interior width	70.5 in
Interior depth	60.1 in
Interior height	75.2 in
Upper bench	70.5 in wide, 20.1 in deep
Lower bench	70.5 in wide, 15.7 in deep
Door glass	23.6 in wide, 68.8 in high, 8 mm tempered with a gasket
Room volume	130 cubic feet

Finnmark's spec sheet and drawing give the interior depth as 57.5 inches where the manual says 60.1. The manual is the document that ships with the sauna, so that is the figure in the table, and the narrower reading is worth knowing if you are planning to the inch.

Where the heat comes from, and why this one is not full spectrum

Finnmark's replacement parts list for the FD-5 names three Spectrum Plus Incoloy elements at 240 volts and 350 watts each, and no carbon panels at all. The infrared only cabins below it pair carbon with Spectrum Plus, which is what their spec sheets mean by full spectrum. Finnmark titles the FD-5 sheet a combination sauna instead, and the combination it means is infrared plus traditional.

The traditional half is a Harvia KIP45B, a 4.5 kilowatt electric stove with a bed of stones. That is more than twice the output of the stove in the two person Trinity, which is what a cabin of 130 cubic feet needs. Pour water on the stones and you get steam.

Construction

Three things about the Finnmark FD-5 Trinity XL you cannot change later

Everything else on a sauna is adjustable. These are set the day it is made.

The two timbers

Inside is Western Red Cedar, which Finnmark specifies for its resistance to mould, bacteria and fungus. Outside is Thermal Plus aspen, which Finnmark describes as treated with heat and steam. The cedar is the surface you sit against; the treated aspen is the shell that faces the room.

The stove, and the room built around it

A cabin designed for a 4.5 kilowatt stove is not an infrared cabin with a heater added. The Trinity XL is nearly five inches taller inside than the infrared only models, the bench runs in two tiers so you can choose your height in the heat, and the door glass carries a gasket. None of that can be retrofitted.

The emitters

Finnmark warrants the Spectrum Plus elements for the life of the sauna. That is the longest term anywhere on the warranty sheet, and these are the parts that decide how a cabin ages.

Compare it against the other hybrid saunas we carry, or against the infrared saunas if the stove is not the point. Our infrared or traditional sauna guide covers what each kind of heat actually does.

Running cost

What a Finnmark FD-5 Trinity XL session costs to run

Worked at 3150 watts of infrared plus the 4.5 kilowatt Harvia, so 7.65 kilowatts with both heaters running, and the US average residential rate of 18.34 cents per kilowatt hour, from the EIA Electric Power Monthly, June 2026. Your own rate is on your bill.

Every input is above, so redo it with your own rate: kilowatts times hours times cents. This is the highest running cost in the Finnmark range, and it is also the only cabin that heats four people and raises steam. Both heaters cycle once the cabin reaches temperature, so a real session costs less than the figures above.

Setup

Assembling the Finnmark FD-5 Trinity XL

It arrives as panels and goes together without specialist tools, but it is the biggest build in the range.

The manual's tool list is a screwdriver and a level. Panels lock to each other and are fixed with two screw lengths Finnmark supplies. The order is back panel, left side, bench support, right side, then the front wall and the top, with the lower and upper benches sliding onto the support. The manual notes that the interior and exterior receptacles on the right panel are not used during assembly.

The Harvia installation is its own step, and it is the step where the second circuit has to already exist. Finnmark's warning is blunt: a minimum of two physically able individuals are required, and the manual says in capitals not to attempt the install without proper professionals or without proper tools and safety equipment. At 575 pounds of panels plus a stove, and panels over six feet wide, two people is a floor rather than a comfortable number. We do not publish an assembly time, because Finnmark does not state one.

Before the first session

Wipe the inside down with a damp cloth to clear any sawdust. Then run the cabin to burn off factory dust before anyone uses it. Check the Harvia's stones are in place and loosely stacked so air can move through them before you switch the stove on for the first time. The serial number plate, which you will want if you ever make a warranty claim, sits underneath the bench.

Reference

Finnmark FD-5 Trinity XL infrared and traditional sauna specifications

Evidence

The Finnmark FD-5 low EMF reading, and what the certificates do not cover

Two documents on this sauna are worth reading in full, and on this model one of them matters for what it leaves out.

The low EMF survey, in full

A third party laboratory in Tempe, Arizona measured Finnmark's Spectrum+ heaters in December 2019 under project PR108242, at three positions on each heater. With everything switched off, the room itself read 0.5304 milligauss. Running, the readings ran from 0.6881 milligauss up to a maximum of 1.1708 milligauss at the third position. So the heater added between 0.2 and 0.64 milligauss over what the room was already producing. The laboratory describes every result as magnitudes lower than the ICNIRP Occupational 2010 test limit.

The 1.1708 maximum is the laboratory's own headline figure, from position three; position one at 100 Hz read 0.7328. Three caveats: the limit is the occupational one rather than the general public one, the survey tested heaters rather than an assembled cabin, and pages four and six of the log name the customer as Influence Sauna rather than Finnmark Designs. The survey covers the Spectrum+ elements, which are the only infrared emitters in this cabin.

The FD-5 is not on the ETL certificates, and we will not say it is

The two Intertek certificates we hold name the FD-1, FD-2, FD-3 and FD-4. The FD-5 is not among them, and neither are the FD-6 or FD-7. We have asked for nothing here to be read as more than it is: the Harvia KIP45B inside this cabin carries its own ETL listing, named on Finnmark's dimension drawing, and that listing covers the stove rather than the sauna. Finnmark separately identifies its Spectrum Plus elements as UL component recognised under file E65554, which covers sheathed heating elements as a component rather than a finished sauna, and that record names the tradename BKR. We do not describe any Finnmark cabin as a UL listed sauna, because that is not what that record says. If a listing elsewhere calls the Trinity XL ETL listed, ask them which certificate names it.

What we do not have

No owner reviews of our own yet, because this is a new listing for us, and no independent test of a finished FD-5 cabin as opposed to its heaters. Finnmark publishes a logged heat-up run for the two person FD-2 but not for this model, and none at all for the traditional side.